Subgroup analysis across different breast cancer subtypes showed that high GZMK expression improved OS in triple-negative breast cancer (TNBC) (n = 404), Luminal A (n = 794), Luminal B (n = 515), and HER2-positive breast cancer (n = 166), with statistically significant differences (P < 0.05) (Figures S3B–E). Here, ERBB2 is linked to triple-negative breast carcinoma.